Familial Hypertrophic cardiomyopathy (HCM) is the most common heritable heart disease in the world, affecting an estimated 1 out of every 500 people. It is characterized by thickening of the heart tissue and is often accompanied by diminished cavity volume and impaired relaxation. Inheritance has been linked to mutations in many genes, most of which relate to the muscle's force generating apparatus (sarcomere). A handful of other genes have also been implicated in the disease, including some that are responsible for sensing and adapting to mechanical loads. The mechanisms by which defects in either force generation or force sensing cause cardiac remodeling in HCM remain largely unknown. Mouse models that express HCM mutations have provided useful insight into HCM pathogenesis. However, model systems for studying the mechanobiology of human cardiomyocytes are sorely needed, because the isoform expression profile of many key proteins to the disease (?MHC in particular) differ greatly between rodents and humans. For instance, it appears that a human mutation substituted into a homologous residue in the murine ?MHC sequence does not replicate the effects of the same mutation in its native (?MHC) context. Fortunately, prospects for studying human HCM mutations in context have improved dramatically with the emergence of induced pluripotent stem cells (iPSCs), which can be derived from patient somatic cells by introduction of stem cell factors. iPSCs can then be used to produce functional cardiomyocytes (iPSC-CMs) for study. We will use this approach to study the functional consequences of sarcomeric and stretch-sensing mutations in HCM. We have recently derived iPSCs from a proband with double heterozygous MYH7[R723C] and MLP[W4R] mutations, who showed a severe asymmetric septal hypertrophy, while the single heterozygote parents were largely asymptomatic. Our preliminary studies revealed that double heterozygote iPSC-CMs were significantly larger than control or single heterozygote iPSC-CMs, recapitulating one of the major HCM defects at the cellular level. Moreover, we showed that the MYH7-R723C mutation caused a prolonged twitch event with slower relaxation, suggesting that these mutant myosin crossbridges have abnormally high affinity for actin. We hypothesize that even a small number of these hyperactive crossbridges remaining attached during diastole would cause excess force to be transmitted to the MLP/z-disk apparatus during ventricular filling (stretch). Stretch causes MLP to translocate to the nucleus, where it activates hypertrophic pathways. Hence, our central hypothesis is that the MYH7-R723C mutation causes derangement of diastolic function that triggers hypertrophy via an MLP- dependent pathway. We will further elucidate the pathogenesis of HCM by determining the mechanism by which the ?MHC and MLP pathways synergistically interact with each other in modulating the disease severity. We will also test our hypothesis by producing engineered heart tissue constructs (EHTs) that can be precisely stretched in vitro. EHTs made from cells of each genotype will be cultured to maturity and then subjected to acute culture under conditions of either constant length or diastolic stretch to mimic ventricular filling.